1. Field of the Invention
The present invention relates to pilot signal strength adjusting methods in mobile communication systems of digital cellular systems DCSs or personal communication services PCSs, in more particularly to, a pilot signal strength adjusting method in mobile communication systems, in which strength of pilot signals generating in common pilot transmit units (hereinafter, referred to xe2x80x9cCPTUxe2x80x9d), which are installed in base stations, is automatically adjustable according to numbers of total calls which are allocated to reference frequencies of the base stations so that hard hand off to different frequencies may be performed smoothly between adjacent base stations.
2. Description of the Conventional Art
In general, as shown in FIG. 1, a mobile communication system is composed of mobile station 10, base station 20, control station 30 and exchange station 40, in order to communicate with anybody, anytime and anywhere by means of radio frequency of space between mobile subscribers or between mobile subscribers and fixed subscribers. Particularly, in cellular mobile communications, total service area is divided into smaller service areas, that is, cells and wireless base stations are positioned. The base stations in the cells are centralized and controlled by the exchange station so that good quality of telephone communication may be continued even though subscribers are removing between the cells.
In the cellular mobile communication, one of the most important characteristics is assuring mobility of teleterminals of the subscribers and location registration and handoff techniques are required for assuring the mobility of the subscribers.
That is, the location registration is a procedure to inform base stations of various characteristics such as identification of current cellular nets positioned by the subscribers and states of the cellular nets. On the other hand, the handoff technique is a technique to switch communication paths between mobile stations and base stations in order to continue telephone conversation even though the mobile stations remove out of the current base station or service area of a current sector.
In the handoff techniques, soft handoff is performed between adjacent base stations, which utilizes a same frequency, and hard handoff is performed between base stations, which utilized different frequencies.
Conventional soft handoff and the hard handoff perform handoff to a same frequency of a same base station or a different frequency of a different base station, when strength of a pilot signal is lowered below a minimum value required to maintain a telephone conversation.
For example, if a mobile station using frequency f2 of base station A, which allocates frequencies f1 and f2, is removing toward base station B which allocates only frequency f1, then the mobile station detects that strength of a pilot signal of frequency f2 of base station A decreases below a threshold value and requests handoff. In response of this handoff request of the mobile station, base station B performs handoff from frequency f2 of base station A to frequency f1 of base station B, thereby forming a new telephone conversation path between the mobile station and base station B.
In the hard handoff system between different frequencies as above, however, base station B allocates only frequency f1, so that only pilot signals of frequency f1 exist and other pilot signals, that is, pilot signals of frequency f2 do not exist. Therefore, even though the mobile station enters into a service area of base station B, it is difficult for base station B to notice direction and extent that the mobile station removes.
Further, since handoff is forced to perform a different frequency of an adjacent base station if strength of a pilot signal of a base station is detected by a mobile station and determined to be lower than a predetermined threshold value by the mobile station, it is difficult to select a proper timing of handoff performance so that call drop occurs frequently. Furthermore, it is difficult to control electric power for the mobile stations since the mobile stations appear in adjacent base stations suddenly, thereby degrading performance of whole system.
Therefore, as shown in FIG. 1, CPTU Common Pilot Transmit Unit 23 is realized to generate separately pilot signals of which frequencies are not allocated by the base station 20. The mobile station 10 detects pilot signals that are generated by the CPTU 23 and performs hard handoff to different frequencies at an optimum timing in traffic and idle states.
At this time, the CPTU 23 is mounted in the base station 20 to perform hard handoff between frequencies and includes a CPCA Common Pilot Control Assembly 21 for controlling strength of common pilot signals and CPTA Common Pilot Transmission Assembly 22 for transmitting common pilot signals toward mobile stations.
That is, in case that a mobile station, which is using frequency f2 among frequencies f1 and f2 which are allocated by base station A, removes toward base station B, which allocates only frequency f2 and generates only f2 pilot signals through CPTU 23, the mobile station detects that strength of f2 pilot signals of base station A is lowered than a threshold value and strength of f2 pilot signal of base station B is higher than the threshold value. The mobile station requests handoff to f2 frequency of base station B, however no sources to handoff toward f2 frequency exist in base station B. Therefore, handoff toward the available f1 frequency of base station B is requested again and hard handoff from f2 frequency of base station A toward f1 frequency of base station B is performed.
The conventional hard handoff as above is successively performed when a cell radius of a common pilot frequency generated at the CPCA 21 in the CPTU 23 is smaller than or equal to a cell radius of a reference frequency of base station. Therefore, the conventional hard handoff is employed in base stations under the state that a cell radius is adjusted by fixing strength of pilot signal of the CPCA 21 equal to or smaller than strength of a reference strength.
However, under the situation that the strength of pilot signal of the CPCA 21 is fixed, if numbers of subscribers, that is telephone conversations are concentrated on the reference frequency of a base station, a cell radius of the reference frequency of the base station becomes smaller than a cell radius of a pilot frequency of CPCA 21. Therefore, due to the change of cell radius of the reference frequency of the base station, a probability that handoff between frequencies apt to be failed increases and telephone conversation of mobile station is apt to be cut, thereby it is impossible to provide continuous conversation service.
In order to resolve the problems of the prior art, strength of pilot signal of CPCA 21 is arbitrarily adjusted in such a manner that the strength is lowered for a time period on which calls are concentrated and recovered for an initial cell radius for a time period on which calls are not concentrated.
However, this has still problems that concentration of calls are continuously watched and adjustment of pilot signal strength of CPCA 21 is manually and complicatedly carried out, thereby accurate adjustment of cell radius of CPCA 21 is rarely achieved.
The present invention is derived to resolve the disadvantages of the conventional techniques and it is an object of the present invention to provide a pilot signal strength adjusting method in a mobile communication system, in which hard handoff may be performed between frequencies regardless of cell radius change of the reference frequency of the base station and that of the pilot signal frequency of CPCA by automatically adjusting strength of pilot signal by increasing and/or decreasing strength of pilot signal of CPTU Common Pilot Transmit Unit by using a sum of calls allocated to a reference frequency of a base station which is installed with the CPTU Common Pilot Transmit Unit, predetermined threshold values of set-up calls, predetermined user-units and predetermined power up/down values, so that continuous telephone conversation of mobile stations may be achieved.
In order to achieve the above objects of the present invention, according to an aspect of the present invention, a pilot signal strength adjusting method in mobile communication system is characterized in that when a mobile station uses service coverage area of a base station which is installed with CPTU Common Pilot Transmit Unit, or calls of mobile stations are concentrated on a reference frequency of the base station which is installed with the CPTU Common Pilot Transmit Unit, strength of pilot signal is adjusted by increasing and/or decreasing power of CPCA Common Pilot Control Assembly which is installed in the CPTU Common Pilot Transmit Unit by using a sum of calls allocated to a reference frequency of a base station which is installed with the CPTU Common Pilot Transmit Unit, predetermined threshold values of set-up calls, predetermined user-units and predetermined power up/down values.
In order to achieve the above objects of the present invention, according to another aspect of the present invention, a pilot signal strength adjusting method in mobile communication system, includes a first step of checking a sum of calls which are allocated to a current reference frequency of a base station if a mobile station is requesting a call set-up to or call release from the reference frequency of the base station, and determining whether the checked sum of calls is larger than a threshold number of set-up calls, a second step of determining whether the call set-up or call release has been performed by a user-unit if the checked sum of calls is determined to be larger than the threshold number of set-up calls, a third step of reducing pilot signal strength by reducing power of CPCA as much as a predetermined power up/down value if the call set-up is determined to have been performed by the user-unit, and a fourth step of increasing pilot signal strength by increasing power of the CPCA Common Pilot Control Assembly as much as the predetermined power up/down value if the call release is determined to have been performed by the user-unit, and repeating from the first step to third step.